The effect of carboxylate groups on the complexation of metal ion with oligopeptides – Potentiometric investigation

•The effect of carboxylic groups depends on their number and position.•The carboxylate groups are the exclusive metal binding sites in the MHL complexes.•The greater is the number of carboxylic groups, the more stable complexes are formed.•None of the studied metal ions induced the deprotonation of amide nitrogens.